[mythtv] Ticket #1049: DVBSignalMonitor needs to be able to monitor NIT/SDT

[Yeasah Pell]

Allan Stirling wrote:

>Daniel Kristjansson wrote:
>  
>
>>On Tue, 2006-06-06 at 23:55 -0400, Yeasah Pell wrote:
>>    
>>
>>>Actually, many cards do much more than a little PLL, and there's a good 
>>>reason -- LOF error on LNBs can be significant (easily greater than your 
>>>tp spacing -- remember, it's some consumer quality ~10GHz oscillator 
>>>powered by some dodgy PCI card's power supply connected by a long piece 
>>>of coax in conditions which are alternately hot and cold, dry and wet), 
>>>so a zigzag frequency walk sometimes needs to happen.
>>>      
>>>
>>I doubt this is as much of a problem as the DVB devs seem to think.
>>I actually asked Kenneth Aafloy to chart the LNB drift and get back
>>to me, he never did. My EE background is in radio engineering, the
>>job of the downconverter portion of the LNB is just to frequency
>>    
>>
>...
>
>I had a spare tuner, so here you go:
>
>    1512 1626758
>   37259 1626760
>   25694 1626761
>   25447 1626762
>   38885 1626764
>   35415 1626765
>   27499 1626766
>   21704 1626768
>   27229 1626769
>   56037 1626770
>   60944 1626772
>   56929 1626773
>   76290 1626774
>  123514 1626776
>   65243 1626777
>   29411 1626778
>   10385 1626780
>    1500 1626781
>    1012 1626782
>     467 1626784
>    1074 1626785
>    9970 1626786
>   11928 1626788
>    5861 1626789
>    7590 1626790
>    9835 1626792
>    6568 1626793
>    4568 1626794
>    5767 1626796
>   13629 1626797
>   18169 1626798
>   17516 1626800
>    6536 1626801
>     149 1626803
>
>This is generated with a modified femon which polls the card 
>  about 5 times a second.
>
>First column is the number of "counts" -  Second column is 
>the DVB real tuned frequency, as reported by the card.
>
>This is tuned to the "Sky One" transponder on Astra 28.2, so 
>it 'should' be centered at 12.285GHz - 10.6Ghz (LOF) =1.685Ghz
>
>Interestingly, there are periodic variations, but it doesn't 
>appear to be only temperature related.
>
>Total swing for this test is 45kHz (0.003%) which is 
>probably not significant. Obviously, longer cable runs, 
>worse power supplies, cheaper LNBs may give worse figures.
>
>Cheers,
>
>Allan.
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>
>  
>
Interesting figures, thanks for the experiment!

For the acquisition search, the rate of change of the LOF is pretty much 
irrelevant though, since acquisition happens over a brief period of time 
-- it's the total error from the LOF's "correct" frequency that 
acquisition has to deal with, since that determines how far you have to 
search just to find the signal that you are looking for. Unless I've 
misinterpreted something, the initial error in the above test is:

1.685GHz - 1.626758GHz = 58.242MHz

Which is so bad it makes me wonder if you actually were locked onto the 
transport you thought you were. I mean, if you express it in absolute 
frequency terms and assume a completely accurate LOF for a second, the 
card ended up at ~12.227GHz which is awfully close to the transponder at 
12.226GHz. If you were on that tp:

1.626GHz - 1626758GHz = 758kHz

Which definitely sounds more like it.

The long-term LOF drift that you show in the chart is something that 
most cards deal with automatically by tracking, but swzigzag isn't in 
the picture for that (it only kicks in after acquisition if signal lock 
is lost for some period), and the tracking capabilities of cards 
presumably are designed only to deal with slow incremental changes in 
frequency, so I think it'd be unlikely that tracking would cause 
significant detuning.